1. Field of the Invention
The present invention relates to a lens barrel, and more particularly to a device for driving and controlling a focusing lens group, which constitutes a lens barrel, to its proper position.
2. Related Art Statement
In recent years, particularly in the field of lens shutter cameras, zoom lenses have enjoyed great popularity and enlargement of the zooming range and reduction in size of zoom lens systems have been progressed. Under such situations, an important problem to be solved to obtain well focused photographs is how to improve the focusing accuracy. For that reason, there have heretofore been presented many proposals relating to driving and control of the focusing position for zoom lenses.
A lens drive unit previously proposed by the applicant in Japanese Patent Laid-Open No. 4-5607 concerns a zoom lens barrel comprising multiple lens groups in which driving and control of the focusing position for a focusing lens group are performed based on the number of shifting pulses from the reference detected position.
The structure of the proposed zoom lens barrel will be described below in detail.
FIG. 5 is a longitudinal sectional view of the proposed zoom lens barrel. A zoom lens barrel 50 is of an inner focusing type zoom lens barrel and has a cylindrical fixed frame 2 with a cylindrical cam ring 1 rotatably fitted around the outer circumference of the fixed frame. A threaded portion 20 is formed in the inner circumferential surface of distal end portion of the cam ring 1, and a focus adjusting ring 21 is screwed into the threaded portion 20. By rotating the adjusting ring 21, the cam ring 1 can be finely moved in the axial direction. A retainer ring 22 is attached to the outer circumferential surface of distal end portion of the fixed frame 2 and is held in abutment with the adjusting ring 21.
Within the fixed frame 2, a cylindrical moving frame 3 is inserted movably in the axial direction. Three roller pins 18 (only one being shown) are attached to the outer circumference of the moving frame 3. Each of the roller pins 18 penetrates through a linear groove 2a formed in the fixed frame 2 and is inserted in a first cam groove 1a formed in the cam ring 1. The linear groove 2a is extended in the axial direction of the fixed frame 2 and the cam groove 1a is formed into a spiral shape. Accordingly, by rotating the cam ring 1, the moving frame 3 is linearly moved in the axial direction. Further, to prevent light from externally entering a gap between the fixed frame 2 and the moving frame 3, a light shielding cloth 23 is attached to the inner circumferential surface of distal end portion of the fixed frame 2 and is held in sliding contact with the outer circumferential surface of the moving frame 3.
Within the moving frame 3, there are disposed first, second and third lens groups 14, 15, 16 which jointly constitute a photographing optical system. Among them, the second lens groups 15 serves as a focusing lens group. Those group lenses will now be explained in more detail. The first lens group 14 is supported in the moving frame 3 by a first group frame 24 screwed into the distal end portion of the moving frame 3. A cylindrical second group frame 4 is fitted in the intermediate portion of the moving frame 3 movably in the axial direction. Three roller pins 17 (only one being shown) are projected on the outer circumference of the second group frame 4. Each of the roller pins 17 penetrates through both a linear groove 3a formed in the moving frame 3 and the linear groove 2a formed in the fixed frame 2 and is inserted in a second spiral cam groove 1b formed in the cam ring 1. Accordingly, by rotating the cam ring 1, the second group frame 4 is linearly moved in the moving frame 3 in the axial direction.
A plurality of focusing guide shafts 10 (only one being shown) are fixed to the second group frame 4 and extended in parallel to the axis of the moving frame 3. A front holder frame 6 and a rear holder frame 7 jointly holding the second lens group 15 are supported by the guide shafts 10 movably in the axial direction of the moving frame 3. A focusing spring 5 is coiled around each of the guide shafts 10 to bias both the front holder frame 6 and the rear holder frame 7 reawardly. A pin 8 projected on the rear holder frame 7 is thereby pressed against the cam surface of a ring-like focusing cam 9. By rotating the focusing cam 9 by drive means (not shown), the front holder frame 6 and the rear holder frame 7 are moved forwardly by the cam 9 against the biasing force of the spring 5 so that the second lens group 15 is shifted to make the focusing operation. Additionally, a shutter blade 25 doubling as an iris diaphragm is provided between the front holder frame 6 and the rear holder frame 7.
A third group frame 12 is fitted in the proximal end portion of the moving frame 3 movably in the axial direction of the moving frame. Three roller pins 19 (only one being shown) are projected on the outer circumference of the third group frame 12. Each of the roller pins 19 penetrates through both a linear groove 3b formed in the moving frame 3 and the linear groove 2a formed in the fixed frame 2 and is inserted in a third spiral cam groove 1c formed in the cam ring 1. Accordingly, upon the cam ring 1 being rotated, the third group frame 12 is linearly moved in the moving frame 3 in the axial direction. Further, a holder frame 11 for holding the third lens group 16 is fitted in and fixed to the inner circumference of the third group frame 12. By adjusting relative positions of the third group frame 12 and the holder frame 11 using a well-known mechanism (not shown), the focus shift during the zooming operation can be compensated for.
FIG. 6 shows respective movements of the lens groups during the zooming operation of the lens barrel 50 in the above-mentioned prior art. Through the zooming operation, the first lens group 14, the second lens group 15 and the third lens group 16, serving as photographing lenses, are moved as shown. FIG. 7 shows the status of focusing control in the lens barrel 50 of the foregoing prior art. Because the pin 8 on the rear holder frame 7 of the second lens group 15 is held in abutment with the cam surface of the focusing cam 9, when a focusing drive motor (not shown) starts rotation while generating shifting pulses, the focusing cam 9 also starts rotating. At some rotational position, an AF switch 13 held in abutment with the outer circumference of the focusing cam 9 is changed from an on-state to an off-state. At this timing t1, pulses applied to the focusing motor start to be counted. Before the above operation, the focusing drive amount fit for the photographing distance of each object is set as the number of pulses (e.g., Pn for the closet object). For example, therefore, if the object is at the closest position, the number of pulses Pn is counted and the focusing operation is stopped at the time (timing t2) the second lens group 15 has been shifted to the in-focus position through the pin 8. After the end of the shutter releasing, the focusing motor is driven again by the number of reset pulses Pn for returning the lens to its initial position (timing t3). In this way, a series of sequential operations inclusive of the focusing driving and the shutter releasing are completed.
For a zoom lens, the focal length fT of a total lens system is generally expressed below; EQU fT=f1.times..beta.2.times..beta.3.times. . . . .times..beta.n(1)
where n is the number of zoom groups, f1 is the focal length of a first lens group, and .beta.i is the magnification of an i-th lens group.
Also, assuming that the spacing between the principal points of the i-th zoom group and the (i+1)-th zoom group is Di, the relationship between a deviation .DELTA.Di of the value Di and a deviation .DELTA.fBi of the focal plane position due to the deviation .DELTA.Di is expressed below: EQU .DELTA.fBi={(.beta.i+1.times..beta.i+2.times. . . . .times..beta.n).sup.2 -1}.times..DELTA.Di (2)
In Equation (2), depending on the spacing Di between the principal points, (.beta.i+1 . . . . .beta.n).sup.2 may have an extremely large value in a tele state and may give a large influence on the deviation .DELTA.fBi.
Also, in the case of an inner focusing type lens barrel, when the i-th zoom group is shifted for focusing, a focusing position deviation .DELTA.fBi' due to such a focusing shift amount .DELTA.Fi is expressed below: EQU .DELTA.fBi'={(.beta.i+1.times. . . . .times..beta.n).sup.2 -(.beta.i.times..beta.i+1.times. . . . .times..beta.n).sup.2 }.times..DELTA.Fi (3)
FIG. 8 is a diagram showing a zooming condition of a zoom lens barrel comprising three lens groups which has characteristics shown in Table 1 below. It is assumed that the focal lengths f1, f2, f3 of first, second and third lens groups are 68.73, 23.60, and 21.13 (mm), respectively. Also, the following Table 2 shows values of .beta.3.sup.2, (.beta.2.multidot..beta.3).sup.2 and so forth which represent values of (magnification).sup.2 in respective zooming modes of the zoom lens barrel. Furthermore, FIG. 9 shows changes in the values of .beta.3.sup.2, (.beta.2.multidot..beta.3).sup.2 and so forth.
TABLE 1 ______________________________________ Mode W S T (wide) (standard) (tele) ______________________________________ Focal length of system (fT) 28.94 54.57 102.5 Spacing between 1st and 2nd 16.46 26.52 31.7 primary points (D1) Spacing between 2nd and 3rd 10.76 3.55 -1.2 primary points (D2) ______________________________________
TABLE 2 ______________________________________ Mode W S T Magnification (wide) (standard) (tele) ______________________________________ .beta.2 .multidot. .beta.3 0.421 0.794 1.491 (.beta.2 .multidot. .beta.3).sup.2 0.177 0.630 2.22 .beta.3 1.352 2.214 3.83 (.beta.3).sup.2 1.83 4.9 14.68 ______________________________________
As will be seen from the above Tables, the influence of the spacing between the primary points of the second and third lens groups upon the deviation of the focal plane position is given by magnification of 13.68 in the tele mode from the above Table 2 and Equation (2), which is about 10 times as much as the magnification of 1.22 corresponding to the influence of the spacing between the primary points of the first and second len groups. It can therefore be said that when the second lens group is used for focusing, the method of directly controlling the spacing between the second and third lens groups is more accurate in driving to get the target focusing position.
Relating to accuracy of the focusing position, it is thus understood that for the zoom lens barrel 50 of the prior art shown in FIG. 5, the spacing D2 between the primary points of the second lens group 15 and the third lens group 16 is more important in point of the focusing accuracy. However, because the focusing operation is performed by managing the number of drive pulses applied to the drive motor as mentioned above, the occurrence of an error in the spacing D2 between the primary points is unavoidable. Of the basic reasons, the first one is that there are errors in the zooming stop positions of the second lens group 15 and the third lens group 16 both driven with the rotation of the cam ring 1. These errors include manufacturing errors of the cams 1b, 1c and wearing of the pins 17, 19. Another factor is an error in the focusing operation itself. This error includes an offset in the timing due to wearing of the AF cam 9 and chattering of the AF switch 13. An additional factor is a dimensional change of the frame components depending on temperature and moisture. Those factors have in no way been eliminated in the foregoing prior art.
FIG. 10 is a graph showing changes required for the spacing D2 between the second and third lens groups with respect to the focal length ranging from the wide mode to the tele mode. More specifically, the spacing D2 required to focus on an object at the infinity (.infin.) for some focal length x is given by a value Dx, and the maximum focusing shift amount of the second lens group 15 for the focal length x is given by Fx. In other words, the point A indicates the focusing position of the second lens group 15 adjusted to focus on .infin. for the focal length x and the point B indicates the focusing position of the second lens group 15 adjusted to focus on the closest object for the same. Likewise, values Dw, Dt represent the spacing D2 in the wide and tele modes, while values Fw, Ft represent changes in the spacings D2 necessary for focusing in the wide and tele modes, respectively. It is thus required that the spacing D2 between the second and third lens groups is controlled following the characteristics shown in FIG. 10.